In injection molding, the plastic is forced into the mold at very high pressure. The mold is then kept at a given temperature while the polymer freezes, the halves are opened and the finished article is ejected. For small parts, lots of molds are often mounted on one machine and the molten plastic is injected into them all simultaneously. Injection molding can produce far more complex items than extrusion, but it isn't a continuous process. 

The finished articles it produces are in their final shape and can be rigid or soft, compact or cellular. Injection blow molding allows hollow articles to be produced.

With overmolding (co-injection), it's possible to produce articles with rigid cores (metal or polymer) by placing the cores in the mold before the plastic is injected.